If AR model provides no "sets", then default should be [] instead of nil to avoid uncaught exception

[jrochkind]

Previously, if the model defined no sets, and a client asked for them, an uncaught exception would be raised, NoMethodError: undefined method detect' for nil`

With a pretty darn confusing stack trace

/Users/jrochkind/code/ruby-oai/lib/oai/provider/model/activerecord_wrapper.rb:123:in `find_set_by_spec'
/Users/jrochkind/code/ruby-oai/lib/oai/provider/model/activerecord_wrapper.rb:99:in `find_scope'
/Users/jrochkind/code/ruby-oai/lib/oai/provider/model/activerecord_wrapper.rb:51:in `find'
/Users/jrochkind/code/ruby-oai/lib/oai/provider/response/list_records.rb:19:in `to_xml'
/Users/jrochkind/code/ruby-oai/lib/oai/provider.rb:444:in `list_records'
/Users/jrochkind/code/ruby-oai/lib/oai/provider.rb:470:in `process_request'
app/controllers/oai_pmh_controller.rb:27:in `index'

By making the default return [] instead of nil, it changes to an appropriate OAI::NoMatchException which is rescued properly by controller to return this response:

<OAI-PMH xmlns="http://www.openarchives.org/OAI/2.0/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.openarchives.org/OAI/2.0/ http://www.openarchives.org/OAI/2.0/OAI-PMH.xsd">
<responseDate>2025-03-04T17:56:12Z</responseDate>
<request>http://localhost:3000/oai</request>
<error code="noRecordsMatch">The combination of the values of the from, until, set and metadataPrefix arguments results in an empty list.</error>
</OAI-PMH>

I don't know if it's ideal, but seems to be what was intended.

I do not use sets, and am not totally sure functionality is working if you do, haven't tracked down a test and the code isn't looking great to me -- But pretty sure this change won't break anything, a sets method that was defined, as modelled, seems like [] would be a legal return value. But maybe not nil.

Got to the bottom of this after local error: sciencehistory/scihist_digicoll#2911